st: Constraint, Poisson, & pseudo R^2

In the Long & Freeze book published by Stata, the authors estimate a Poisson
regression model using the constraint command to control for unequal
exposure. It produces the same coefficients as when using exposure or offset, but
the pseudo-R^2 value is substantially larger when using the constraint option
(p. 265). However, when I use the constraint option, Stata does not produce a
pseudo-R^2 value. Does anyone know whether there has been a change made to
the program by Stata?
Thanks in advance.
Bob Kaminski
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